Abstract: High-frequency protection fault signals in transmission line relay protection suffer from delay and attenuation during transmission, leading to signal offset and energy loss. Furthermore, the different fault locations result in phase differences in the fault current. The time-frequency localization characteristics of wavelet transform are well-suited for multi-resolution analysis and singularity detection of complex signals. This paper utilizes the db wavelet to decompose the fault current in phase-difference high-frequency protection and uses this to identify internal and external faults. An example simulation using Matlab demonstrates the feasibility and effectiveness of the proposed method. Keywords: Phase-difference high-frequency protection; wavelet analysis; relay protection; fault identification 1. Introduction To meet the needs of rapidly developing economic construction, China's power system is expanding, and the requirements for power supply reliability are increasing. To better ensure safe and economical operation, the operation of the power system increasingly relies on relay protection. On high-voltage and ultra-high-voltage lines, to ensure stable operation of the power system and reduce the damage caused by faults, relay protection devices must be able to instantly disconnect faults at any point on the protected line. Under these conditions requiring rapid operation across the entire line, only longitudinal protection can be used. Because longitudinal differential protection theoretically possesses absolute selectivity, it is the primary protection method for high-voltage and ultra-high-voltage transmission lines. With the development of relay protection technology, fiber optic longitudinal differential protection is increasingly widely used in power systems. However, many 110–220kV transmission lines still use phase-difference high-frequency protection because this type of protection does not react to system oscillations, will not malfunction during non-full-phase operation, and its operating state is unaffected by voltage circuit breaks. Therefore, it is suitable for lines equipped with single-phase reclosing and integrated reclosing. The basic principle of phase-difference high-frequency protection is to determine internal and external faults by comparing the phase relationship of the operating currents I_m and I_m at both ends of the protected line. Ideally, when a fault occurs inside the protection zone, the currents at both ends of the line are in phase, and the protection devices at both ends trip the circuit breakers; while when a fault occurs outside the protection zone, the currents at both ends are in opposite phases (phase difference of 180°), and the protection devices at both ends should not operate. However, in reality, due to various factors, the phase difference of the operating currents at both ends is not 180°. Wavelet analysis, with its time-frequency localization property, can effectively analyze non-stationary signals. Using wavelet transform to analyze fault currents can identify internal and external faults. For details, please click: Fault Identification of Phase Difference Protection Based on Wavelet Analysis